st/mesa: don't draw instead of asserting in transform feedback

if we get a request to take the count from feedback, but there
is no buffer to take it from, just draw as if we got 0 vertices
so nothing.

This fixes this assert killing the ogl conform, and a piglit
test I've sent.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/state_tracker/st_cb_xformfb.c

@@ -191,7 +191,6 @@ st_transform_feedback_get_draw_target(struct gl_transform_feedback_object *obj)
       }
    }
 
-   assert(0);
    return NULL;
 }
 
@@ -211,14 +210,17 @@ st_end_transform_feedback(struct gl_context *ctx,
 }
 
 
-void
+bool
 st_transform_feedback_draw_init(struct gl_transform_feedback_object *obj,
                                 struct pipe_draw_info *out)
 {
    struct st_transform_feedback_object *sobj =
          st_transform_feedback_object(obj);
 
+   if (sobj->draw_count == NULL)
+      return false;
    out->count_from_stream_output = sobj->draw_count;
+   return true;
 }
 
 

src/mesa/state_tracker/st_cb_xformfb.h

@@ -38,7 +38,7 @@ struct pipe_draw_info;
 extern void
 st_init_xformfb_functions(struct dd_function_table *functions);
 
-extern void
+extern bool
 st_transform_feedback_draw_init(struct gl_transform_feedback_object *obj,
                                 struct pipe_draw_info *out);
 

src/mesa/state_tracker/st_draw.c

@@ -242,7 +242,8 @@ st_draw_vbo(struct gl_context *ctx,
       /* Transform feedback drawing is always non-indexed. */
       /* Set info.count_from_stream_output. */
       if (tfb_vertcount) {
-         st_transform_feedback_draw_init(tfb_vertcount, &info);
+         if (st_transform_feedback_draw_init(tfb_vertcount, &info) == false)
+            return;
       }
    }